1. Field of the Invention
The present invention relates to methods for efficiently controlling access to a communication system. More particularly, the present invention relates to a method for preventing the occurrence of overload conditions in a multi-carrier communication system by controlling the amount of voice and data users having access to the communication system.
2. Description of the Prior Art
Wireless communication systems, and in particular, multi-carrier systems such as Code Division Multiple Access (CDMA) include many communication channels through which subscribers of such systems communicate with each other and with the system. CDMA systems and other similar communication systems have a certain inherent capacity. That is to say, such systems are limited by the amount of communication channels that can be made available to subscribers of such systems. The capacity of a communication system is the amount of information per unit time (i.e., information rate) that can be transferred within the system while at the same time maintaining an acceptable quality of communications (i.e., quality of service or simply QoS). The operator of the communication system often defines the acceptable QoS. However, recent systems are now heading towards levels of acceptable QoS required, and thereby defined, by the given subscriber. System capacity is typically related directly to the number of subscribers using the given system where the more subscribers there are using the system, the higher the information rate.
Topology of a typical CDMA system involves numerous cells each of which form a sector that is simply a particular geographical area or terrain that is covered by the system. Such cells are usually symbolically represented having borders that form hexagons where the hexagons are areas of coverage within which subscribers located in the same cell communicate with the cell. Each cell has system equipment that is owned and controlled by a system operator. The system equipment is used by the system to admit subscribers to the system subject, of course, to system capacity and QoS requirements. Thus, subscribers of the system gain access to the communication system for communicating with each other and/or with the system. At least part of the system equipment is commonly located at a base station. Some of the system equipment at the base stations is radio frequency (RF) transmitters and receivers for transmitting and receiving communication signals.
Other system equipment that can also be located at a base station provides the operations, administration, and maintenance (OA&M) services typically associated with communications equipment. For example, allocating communication channels for subscribers, and giving subscribers access to the communication system are some of the services provided by the OA&M equipment. A given subscriber provided access to the communication system can communicate with other subscribers via the base station corresponding to the cell in which the given subscriber is located.
For CDMA communication systems, a subscriber gains access by making a request to system equipment usually located at a base station. For example, a given subscriber in the cell will make a request to that cell's base station to have radio access to the communication system. System equipment at that base station will receive the request and decide whether to provide that subscriber with access to the communication system. That subscriber and related base station (as well as other base stations and subscribers) communicate via communication channels called forward links and reverse links. The forward link is a communication channel through which base station transmits communication signals to subscriber. The reverse link is a communication channel through which subscriber transmits communication signals to base station. Thus, each subscriber has a forward link and a reverse link that it uses to communicate with system equipment and/or with other subscribers of the communication system.
Typically, the system decides to provide access to a subscriber by performing a power level analysis that attempts to maintain the QoS among subscribers. The system could continue to admit subscribers and thus keep increasing its information rate. However, at some point an overload condition will occur that causes QoS to be degraded. An overload condition occurs when QoS for existing voice and/or data calls drops below an acceptable level set by the system operator. Often, the acceptable level is set as a threshold below the system's ultimate capacity. One example of an overload condition is when a cell communicates with a relatively large number of subscribers such that the system cannot meet the desired signal to noise ratio requirement. In order to balance the load on the system, several prior art attempts have been made utilizing thresholds as triggering mechanisms.
U.S. Pat. No. 4,670,899 granted to Brody et al. on 2 Jun. 1987 discloses load balancing for cellular radiotelephone systems. In Brody et al., balancing of loading of cells in a cellular mobile radio telephone system is performed by periodically determining the channel utilization of each cell, computing a representative voice channel occupancy level, and attempting to hand-off calls from cells with higher voice channels occupancy levels to adjacent cells with lower voice channel occupancy levels. Voice channel occupancy levels of cells are measured and compared with threshold values, and the results of the comparisons are used to direct cells to enter predetermined states. In one state, directing cells prevent complete cell blockage to hand-off calls to adjacent cells. In another state, voice channels are preserved for incoming hand-offs by directing the cell to deny access to mobile transceivers initiating new calls. Cells may assume a combined stage wherein both of these functions are performed simultaneously. Cells are selected as hand-off candidates for hand-offs initiated to more evenly distribute loading throughout the cellular system in accordance with cell state (i.e., voice channel occupancy level) and measured signal strength at the cells of the calls attempted to be handed off.
U.S. Pat. No. 5,241,685 granted to Bodin et al. on 31 Aug. 1993 discloses a load sharing control for a mobile cellular radio system. In Bodin et al., load balancing in a mobile cellular radio system is achieved by dynamically moving the borders between any two cells such that an overloaded cell becomes smaller and the neighboring cell larger. This is achieved by lowering the entering signal strength threshold for handoff to the neighboring cell and/or increasing the entering signal strength threshold for handoff from the neighboring cell. Thresholds are unique for any two cells.
U.S. Pat. No. 6,456,850 granted to Kim et al. on 24 Sep. 2002 discloses a method for preventing overload conditions in a communication system. The communication system performs a call load analysis and admits a subscriber requesting admission (or responding to a page) to the system based on the result of the call load analysis. The call load analysis is based on the signal to noise power ratios of all subscribers already admitted to the communication system. External jammer signals that interfere with subscriber signals thus reducing the coverage of the communication system do not affect the call load analysis. The jammer signals therefore, do not hinder an efficient use of the capacity of the communication system. A threshold value is established and such threshold value is compared to an average call load value calculated from a plurality of instantaneous call load values. When the calculated average call load value is substantially equal to or above the established threshold, no subscribers are admitted to the communication system; subscribers are admitted when the calculated average call load value is below the established threshold. Individual subscribers whose contributions to the average call load value are deemed significant (tending to cause or actually causing overload conditions) are identified and removed from the system.
U.S. Pat. No. 6,574,474 granted to Nielsen on 3 Jun. 2003 discloses load balancing for cellular and wireless systems. In Nielsen, a system is disclosed for assigning a cellular site to provide wireless telephone service to a mobile telephone set based upon two criteria. This invention assigns a cellular site to provide service based upon a primary criteria, such as signal strength between the cellular site and the mobile telephone set. A secondary criteria, such as load of the cellular site, is then checked to determine whether a threshold has been exceeded by the assignment of the cellular site to provide service. If the threshold for the secondary criteria is exceeded by the assignment, another cellular site is assigned to provide service to the mobile telephone set.
In a wireless system supporting both power-controlled voice service (or generally any other real-time, circuit-switched service) and rate-controlled data service, there is a need to ensure the QoS or outage criteria of both voice and data are met, while maximizing the overall system capacity. Such wireless systems include the evolution of the CDMA2000 1x system that is also known as 1xEV-DV (1xEVolution for Data and Voice). CDMA operators around the world are upgrading their wireless networks from IS-95 to 1xEV-DV in order to cope with the ever-increasing demand of wireless communication needs and to offer new services such as high-speed access to the Internet. However, offering services to voice and high rate data users simultaneously is quite challenging. Enhancements to the 1xEV-DV system are needed to offer packet data services at greater and greater rates (>2 Mbps) while simultaneously supporting voice users. QoS and load balancing are two major related concerns.
Moreover, the above-referenced prior art does not address multi-carrier communication systems. What is needed therefore is method for preventing the occurrence of overload conditions in a multi-carrier communication system by controlling the amount of voice and data users having access to the communication system in order to provide enhanced QoS and load balancing.